Heretofore, the circuit substrate having incorporated therein a resistance material layer has been generally provided in the form of a laminate comprising an electrically insulating material layer as supporter, a resistance layer bonded to the surface of the insulating layer, and a highly conductive material layer bonded to the resistance layer. The desired resistance circuit pattern has been prepared by forming, in conformity with a prescribed circuit pattern, an insulating region (obtained by the removal of all the layers deposited on the insulating layer), a resistance layer (obtained by the removal of only the layer of highly conductive material), and a conductor region (obtained without removal of any layer; normally having the surface thereof plated with a thin layer of a noble metal such as gold) by the subtractive process (mask-etching process) or by directly forming the resistance elements and the conductive elements (as electrodes) by printing on the insulating layer through screen printing plates having prescribed patterns.
The material used for the resistance layer in the art of this field comprises carbon sources, metal oxide sources, metal sources, or mixtures thereof. The method comprising printing a paste-like material obtained by mixing, for example, carbon particles with various resin components, the method comprising carbonizing and depositing various hydrocarbon compounds under various conditions, and the method comprising depositing or sputtering a metal or an alloy comprising at least two metals are known as the method for forming the resistance layer from such a material.
In the case of the method comprising printing the paste-like material, the resistance value conferred upon the resistance element is not easily controlled, the resistance value greatly varies over the entire region of the circuit board and the properties of the resistance elements are not satisfactory. In the method comprising depositing or sputtering a metal or alloy, the control of the resistance value is difficult and the facilities required for the operation of this method become expensive.
In recent years, therefore, a great attention has been made to the method which can produce the resistance layer efficiently and inexpensively by plating in a stable manner with a large area. Japanese Patent Application (OPI) No. 73762/1973 (corresponding to U.S. Pat. No. 3,808,576) discloses a method which produces a resistor comprising nickel-phosphorus alloy by electroplating and Japanese Patent Application (OPI) No. 71513/1975 (corresponding to U.S. Pat. No. 3,857,683) discloses a method which forms a resistance layer comprising various binary alloys other than the alloy mentioned above by electroplating. The alloys mentioned above, however, have various defects in the properties and processability as materials for the desired resistance elements.
In the circuit substrate having a resistance layer of thin metal film, the resistance elements having the desired resistance value per unit area (sheet resistance value) can be obtained by decreasing the film thickness. The thickness of the metal film, however, has its own limit because the microscopic uniformity of the metal film tends to be difficult to obtain as the thickness of the metal film decreases. The sheet resistance value of the nickel-phosphorus alloy which can be industrially produced is at most 100 ohms per square and a film of this alloy having a greater sheet resistance value cannot be obtained. In addition, the steps of the subtractive process involve serious drawbacks.
In the subtractive process, the first step is to apply a photoresist to the entire surface of a copper foil (highly conductive material) on the circuit substrate. Then, the resulting photoresist coating is exposed to light through a photomask of a pattern which allows the photoresist to remain intact in the portions intended to form a resistance element and a conductor portion, followed by developing. Unnecessary copper and also resistance layer are sequentially etched out with the respective etching solution to form an insulating region. Thereafter, the circuit substrate thus treated is exposed to light through a photomask of a pattern which allows the photoresist to remain intact only in the portion intended to form the conductor portion, followed by developing. Then, the exposed portion of the copper foil is etched out (for the formation of the resistance element) to obtain the desired circuit board (wherein the photoresist still remains on the surface of the conductor portion).
In the procedure described above, in the removal by etching of the portion of copper foil corresponding to the resistance region, it is imperative for the material of the resistance layer to be stable enough against the etching solution and to be not substantially etched.
Unfortunately, it has been ascertained that the resistance element which is formed of the nickel-phosphorus alloy exhibits poor etching selectivity relative to the copper foil, so that the resistance element is partially etched out when the copper foil on the resistance element is removed by etching and, consequently, the resistance value of the resistance element is greatly increased. In other words, the initially predetermined resistance value does not agree to the resistance value of the resistance element after processing.
Although the various binary alloys disclosed in Japanese Patent Application (OPI) No. 71513/1975 have been proposed as products which can provide higher sheet resistance value than that of a plating film of the respective single metal in the same plating conditions, they have not yet been industrially employed for the following reasons. Those alloys have the problem that it is difficult to balance the increase of the sheet resistance value due to the decrease of the thickness of the resistance layer with the various properties of the resistance element such as the etching selectivity and also the problem that it is extremely difficult to produce a plating film of alloy comprising constant compositions and free from scattering of the resistance value from a plating bath in a stable manner.
A tin-nickel alloy is proposed as a material for the resistance element different from the above-described various alloys. This alloy has various advantages that a thinner film can be formed as compared with the above-described alloys so that the sheet resistance value in the range of at most about 300 to 400 ohms per square can be obtained, the etching selectivity is improved and the uniform electrodepositing property when the film is formed by electroplating is excellent.